Electronic component with cavity fillers made from thermoplast and method for production thereof

ABSTRACT

An electronic component and a method for fabricating it is disclosed, where the component comprises a semiconductor chips which has flip-chip contacts. These contacts are fixed on a rewiring substrate, the interspace between the rewiring substrate and the semiconductor chip being filled with a thermoplastic. The glass transition temperature of the thermoplastic is above the highest operating test temperature of the component and below the melting temperature of the solder material for external contacts.

FIELD OF THE INVENTION

The invention relates to an electronic component with a semiconductorchip which has flip-chip contacts and is fixed on a rewiring substrate,and also to a method for fabricating it.

BACKGROUND

Electronic components with flip-chip contacts and a rewiring substrateare packaged in a plastic package made of thermosetting plastics. Whenexternal contacts are attached to external contact pads on the rewiringsubstrate of such electronic components or when the finished externalcontacts are soldered onto a circuit carrier, some of these electroniccomponents fail unexpectedly, even though their operability has beensuccessfully tested beforehand for temperature cycles between a topoperating test temperature of approximately plus 150° C. and a bottomoperating test temperature of approximately minus 50° C.

SUMMARY

The invention provides an electronic component with underfillers made ofthermoplastics, and method for fabricating the electronic component.

It is an object of the invention to specify an electronic component anda method for fabricating it which increases the reliability ofelectronic components.

This object is achieved by means of the subject matter of theindependent claims. Advantageous developments can be found in thedependent claims.

The invention provides an electronic component having a semiconductorchip which has flip-chip contacts on its active top side which are fixedto contact pads on a rewiring substrate. This fixing can be obtained bymeans of a solder connection and/or using a conductive adhesive. Theinterspace formed between the rewiring substrate and the semiconductorchip by the flip-chip contacts has a thermoplastic as an underfiller.The glass transition temperature of this thermoplastic used as anunderfiller is below the melting temperature of the solder material ofthe external contacts on the electronic component.

Such a component has the advantage that the instances of failure of theelectronic components are reduced when soldering external contacts ontoexternal contact pads and when soldering external contacts on theelectronic component onto circuit carriers. When a thermoplastic isadded which exceeds its glass transition temperature and softens duringsoldering operations in the region of the external contacts and, uponreaching the soldering temperature, changes to a liquid state, theeffect achieved is that stresses resulting from vapor phase formation inthe case of duraplastic materials as plastic package compound are toneddown. The softened thermoplastic is able to deform plastically and henceto yield without destroying the joints between flip-chip contacts on thesemiconductor chip and contact connection pads on a rewiring substrate.Hence, the failure rate when soldering external contacts or whensoldering onto circuit carriers is reduced.

In all cases, however, the glass transition temperature and hence thesoftening point is above the highest operating test temperature forelectronic components, which may be between 70 and 150° C., depending onthe area of application. Consumer components are not tested so hard, andhence are tested at a lower maximum operating test temperature thancommercial components, such as electronic components for automotiveengineering, which are cyclically subjected to a maximum operating testtemperature of 150° C. during the operating test. The glass transitiontemperature for the thermoplastic provided as an underfiller then alsoneeds to be chosen to be correspondingly higher.

A further advantage of this electronic component is that the package nolonger needs to be predried before each soldering process in order toexpel moisture, since a higher level of moisture can be tolerated when athermoplastic is used as underfiller, without the joint or the structureof the component being destroyed.

The thermoplastic used may be one of the materials from the groupcomprising polyamide, polyacetal, polycarbonate, polyethylene,polypropylene, polyethylene terephthalate or mixtures thereof.Particularly by mixing these thermoplastics, it is possible to set thedesired softening temperature range and melting temperature range. Thisensures that the thermoplastic has the same strength at the maximumoperating test temperature as at room temperature, especially since theglass transition temperature for the thermoplastic is not reached untilabove this point.

In contrast to soldering, where only parts of an electronic componentare heated and only parts of it can reach critical temperatures, for theoperating test the electronic components are exposed fully to a maximumoperating test temperature, which may be 150° C. At such a temperature,the thermoplastic needs to have the same consistency and strength as atroom temperature. Only at the much higher soldering temperature of theexternal contacts, which may reach 250° C., does the thermoplastic asunderfiller have a plastic compliance or liquid properties which preventthe components of the electronic component, particularly thesemiconductor chip, the flip-chip contacts and the contact pads on therewiring substrate, from being damaged or destroyed, or theirinterconnections from being broken.

A plastic package containing the semiconductor chip and the flip-chipcontacts may have a thermoplastic with the same glass transitiontemperature as the underfiller. This has the advantage that the plasticpackage and the underfiller can be introduced in a single transfermolding step.

Before they are introduced, the flip-chip contacts can be securely fixedon appropriate contact pads on the rewiring substrate, especially sincethe inventive design of the electronic component allows the package tobe fabricated without the need for the semiconductor chip to be pressedonto appropriate contact pads on the rewiring substrate by plastic filmor plastic layer before it is packaged with its flip-chip contacts.

The plastic package may also comprise a thermoplastic with a glasstransition temperature which is above the melting temperature of thesolder material for the external contacts. In this case, when certainparts have reached the soldering temperature, only the thermoplasticused as an underfiller (which thermoplastic softens at a lowertemperature) will yield as it softens or becomes liquid. However, thisplastic yielding by the underfiller is sufficient to prevent theconnections between semiconductor chip and rewiring substrate from beingdamaged or destroyed. In this case, two successive transfer moldingprocesses are required in order to apply the two differentthermoplastics firstly as an underfiller and then as a plastic package.

Advantageously, the thermoplastic may be in a liquid state in atemperature range between 200° C. and 220° C. In such a liquid state,the thermoplastic is sufficiently compliant for stresses resulting fromthe formation of water vapor to be compensated for. In addition, thistemperature range is clearly above a maximum operating test temperatureand below a soldering temperature for the external contacts.

A method for fabricating an electronic component has the followingmethod steps: first, a rewiring substrate with contact pads on its topside and external contact pads on its underside is fabricated. In therewiring substrate, the external contact pads on the underside areconnected to the contact pads on the top side of the rewiring substratevia through holes and via rewiring lines. In addition, a semiconductorchip using flip-chip technology is fabricated with flip-chip contacts onits active top side.

If both the rewiring substrate and the semiconductor chip with flip-chipcontacts are available, then the flip-chip contacts are put onto therewiring substrate and are electrically connected to the contact pads.Finally, the interspace between the active top side of the semiconductorchip and the top side of the rewiring substrate can be filled with anunderfiller made of thermoplastic.

This method has the advantage that filling the interspace between thesemiconductor chip and the rewiring substrate does not involve the useof a thermosetting plastic which, particularly when soldering externalcontacts or when soldering the external contacts onto a circuit carrier,might damage or destroy the connection between semiconductor chip andrewiring substrate when moisture occurs.

The flip-chip contacts may be soldered onto the contact pads on therewiring substrate or may be fixed using a conductive adhesive beforethe thermoplastic is introduced as underfiller. Since this method steptakes place even before the underfiller is introduced, a secure,reliable electrical connection can be provided by means of the flip-chipcontacts to the rewiring substrate and hence to the external contactpads on the rewiring substrate.

The underfiller may be applied with appropriate heating using dispersiontechnology, which means that it is possible to dispense with ahigh-pressure mold. If the plastic package is made of the same materialas the underfiller, then the plastic package can be produced at the sametime as the underfiller. In this case, it is advantageous to apply thethermoplastic using injection-molding technology, which means that it ispossible to underfill and mold the plastic package in one step.

Before the thermoplastic is introduced onto the top side of the rewiringsubstrate, it is heated to a processing temperature above the maximumoperating test temperature and below the melting temperature of thesolder material for external contacts. Preferably, provision is made forthe thermoplastic to be heated to temperatures between 200 and 220° C.before it is applied to the rewiring structure.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be explained in more detail with reference to theaccompanying figures.

FIG. 1 illustrates a schematic cross section through an electroniccomponent which has been put onto a circuit carrier.

FIG. 2 illustrates a schematic cross section through a critical portionof an electronic component.

FIG. 3 illustrates a schematic cross section through an electroniccomponent with a plastic package which has been put onto a circuitcarrier.

DETAILED DESCRIPTION

FIG. 1 illustrates a schematic cross section through an electroniccomponent 1 which has been put onto a circuit carrier 12 for aelectronic circuit by its external contacts 10. The electronic component1 essentially comprises two main components, namely a semiconductor chip2 and a rewiring substrate 6.

The rewiring substrate 6 essentially has five layers. Starting from itstop side 13, the five layers are staggered down to the underside 15 asfollows: an upper solder resist layer 19, an upper rewiring layer 20, anelectrically insulating core plate 21, a lower rewiring layer 22 and alower solder resist layer 23. The lower solder resist layer 23 coversthe underside 15 of the rewiring substrate 6 as far as external contactpads 13, on which external contacts 10 in the form of solder balls aresoldered. The external contact pads 14 are part of the lower rewiringlayer 22, which is electrically connected to the upper rewiring layer 20by means of through holes 16. The upper solder resist layer 19 leavesonly the contact pads 5 on the upper rewiring layer 20 free of solderresist.

The semiconductor chip 2 has an active top side 4 and a passive reverseside 24. The active top side 4 has contact pads 18 arranged on it whichcarry flip-chip contacts 3 in the form of solder balls or bumps. The twomain components of the electronic component 1 are electricallyinterconnected by means of the flip-chip contacts 3 on the semiconductorchip 2 and the contact pads 5 on the upper rewiring layer 20 of therewiring substrate 6. An interspace 7 which forms between the active topside 4 of the semiconductor chip 2 and the top side 13 of the rewiringsubstrate 6 is filled with a thermoplastic 8.

This thermoplastic 8 or the mixture of thermoplastics has a glasstransition temperature between 155° C. and 250° C. The critical phasewhen assembling an electronic component 1 of this type and when addingan electronic component 1 of this type to the top side of a circuitcarrier 12 is when the external contacts 10 are heated to solderingtemperature.

FIG. 2 illustrates a schematic cross section through a critical portionof an electronic component 1. This critical portion is the interspace 7between the active top side 4 of the semiconductor chip 2 and the topside 13 of the rewiring substrate 6. This interspace has a permanentconnection in the form of flip-chip contacts 3 between the contact pads18 on the semiconductor chip 2 and contact pads 5 on the upper rewiringlayer 20 of the rewiring substrate 6. Since plastics are hygroscopic,they absorb moisture when there are interlayer deposits.

When soldering external contacts (not shown in FIG. 2) of the electroniccomponent, vapor bubbles 25 may form and exert a pressure on the topsides of the rewiring substrate 6 and the semiconductor chip 2, whichare connected by means of the flip-chip contacts 3. An underfiller 9made of the thermoplastic 8 filling the interspace 7 may yield to thispressure, especially since it is plastically compliant or liquid in theregion of the soldering temperature and may thus alleviate the stressresulting from a vapor bubble 25 of this type.

The risk of the electrical connection of the flip-chip contacts 3 beingseparated from the contact pads 5 on the rewiring substrate 6 islessened. Rather, the electrical connection is maintained both whensoldering the external contacts to the external contact pads, as areshown in FIG. 1, and when soldering the electronic component onto acircuit carrier.

FIG. 3 illustrates a schematic cross section through an electroniccomponent 1 with a plastic package 11 which has been put onto a circuitcarrier 12. Components having the same functions as in the precedingfigures are identified by the same reference symbols and are notdiscussed separately.

The difference between this electronic component 1 and the component 1shown in FIG. 1 is that the passive reverse side of the semiconductorchip 2 is not freely accessible as in FIG. 1, but rather is covered witha plastic package 11. In this embodiment of the invention shown in FIG.3, this plastic package 11 comprises the same thermoplastic 8 as thatfrom which the underfiller 9 is already formed. The underfiller 9 andthe plastic package 11 were put on in a single transfer molding step. Toavoid possible partial deformation or melting of the plastic package 11during soldering, the plastic package 11 can be cooled to some extentduring the soldering operation.

1-9. (canceled)
 10. A method for fabricating an electronic component,comprising: providing a rewiring substrate, having contact pads on itstop side; providing a semiconductor chip in flip-chip technology havingflip-chip contacts on its active top side; applying and electricallyconnecting the flip-chip contacts to the contact pads of the rewiringsubstrate; substantially filling an interspace between the active topside of the semiconductor chip and the top side of the rewiringsubstrate with an underfiller which comprises a thermoplastic.
 11. Themethod of claim 10, comprising soldering the flip-chip contacts onto thecontact pads before the underfiller is introduced.
 12. The method ofclaim 10, comprising: applying, essentially simultaneously with theintroduction of the underfiller, a plastic package made of the samethermoplastic material in order to package the semiconductor chip. 13.The method of claim 10 comprising: heating the thermoplastic, prior tobeing applied to the rewiring substrate, to temperatures below themelting temperature of the solder material for external contacts,preferably to temperatures between 200° C. and 220° C., and is changedto a liquid state.
 14. The method of claim 10, comprising: applying thethermoplastic as the underfiller using dispersion technology.
 15. Themethod of claim 10, comprising: Applying the thermoplastic asunderfiller using injection-molding technology.
 16. A method forfabricating an electronic component, comprising: providing a rewiringsubstrate, having contact pads on its top side; providing asemiconductor chip in flip-chip technology having flip-chip contacts onits active top side; applying and electrically connecting the flip-chipcontacts to the contact pads of the rewiring substrate; substantiallyfilling an interspace between the active top side of the semiconductorchip and the top side of the rewiring substrate with an underfillerwhich comprises a thermoplastic; soldering the flip-chip contacts ontothe contact pads before the underfiller is introduced; and heating thethermoplastic, prior to being applied to the rewiring substrate, totemperatures below the melting temperature of the solder material forexternal contacts, preferably to temperatures between 200° C. and 220°C., and is changed to a liquid state.
 17. The method of claim 16,comprising: applying, essentially simultaneously with the introductionof the underfiller, a plastic package made of the same thermoplasticmaterial in order to package the semiconductor chip.
 18. The method ofclaim 16, comprising: applying the thermoplastic as the underfillerusing dispersion technology.
 19. The method of claim 16, comprising:Applying the thermoplastic as underfiller using injection-moldingtechnology.
 20. An electronic component comprising: a semiconductor chipwhich has flip-chip contacts on its active top side which are fixed oncontact pads on a rewiring substrate; an underfiller within theinterspace between the rewiring substrate and the semiconductor chipwhich arises as a result of the flip-chip contacts, the underfillercomprising a thermoplastic whose glass transition temperature is below amelting temperature of a solder material of external contacts of theelectronic component.
 21. The electronic component of claim 20, whereinthe thermoplastic comprises at least one material from the groupcomprising polyamide, polyacetal, polycarbonate, polyethylene,polypropylene, polyethylene terephthalate or mixtures thereof.
 22. Theelectronic component of claim 20, wherein a plastic package for theelectronic component comprises a thermoplastic having the same glasstransition temperature as the underfiller.
 23. The electronic componentof claim 20, wherein the thermoplastic is in a liquid state in atemperature range between 200° C. and 220° C.
 24. An electroniccomponent comprising: a rewiring substrate having an upper solder resistlayer, an upper rewiring layer, an electrically insulating core plate, alower rewiring layer and a lower solder resist layer; a semiconductorchip which has flip-chip contacts on its active top side which are fixedon contact pads on the rewiring substrate; an underfiller within theinterspace between the rewiring substrate and the semiconductor chipwhich arises as a result of the flip-chip contacts, the underfillercomprising a thermoplastic whose glass transition temperature is below amelting temperature of a solder material of external contacts of theelectronic component.
 25. The electronic component of claim 24, whereinthe thermoplastic comprises at least one material from the groupcomprising polyamide, polyacetal, polycarbonate, polyethylene,polypropylene, polyethylene terephthalate or mixtures thereof.
 26. Theelectronic component of claim 25, wherein a plastic package for theelectronic component comprises a thermoplastic having the same glasstransition temperature as the underfiller.
 27. The electronic componentof claim 24, wherein the thermoplastic is in a liquid state in atemperature range between 200° C. and 220° C.
 28. The electroniccomponent of claim 24, further comprising: a circuit carrier having anelectronic circuit, coupled to the electronic components via externalcontacts.
 29. An electronic component comprising: a semiconductor chipwhich has flip-chip contacts on its active top side which are fixed oncontact pads on a rewiring substrate; means for underfilling, within theinterspace between the rewiring substrate and the semiconductor chipwhich arises as a result of the flip-chip contacts, the underfillermeans comprising a thermoplastic whose glass transition temperature isbelow a melting temperature of a solder material of external contacts ofthe electronic component.
 30. The electronic component of claim 16,wherein the thermoplastic comprises at least one material from the groupcomprising polyamide, polyacetal, polycarbonate, polyethylene,polypropylene, polyethylene terephthalate or mixtures thereof.